Electroplating comprises the adherent electrodeposition of a layer or coating of one conductive material onto another. A plating bath is generally set up by dissolving an amount of a metal salt or other compound in an aqueous solvent. The salts dissociate into their respective metal cations and anions, and the cations are reduced to metal on the surface of a conductive object which has been made the cathode in the bath. The anode is generally fabricated of the same metal which is to be deposited, thus assuring a constant supply of ions and negating the chance for contaminating the bath.
The above simplistic description fails to indicate, however, the myriad complexities which go into determining the smoothness (and, hence, often the utility) of the electroplated coating, and a variety of substrate preparation steps (cleaning, rinsing, acid dipping, etc.) are frequently implemented as part of the overall electroplating process. Failure to properly treat the substrate can often result in the failure to achieve a bright, shiny, smooth coating, a feature which not only is of great practical significance when, for example, considering metal-on-metal contact, wear and longevity, but which is also of obvious significance in creating an attractive product for marketing purposes.
Even with proper substrate preparation, dull or matte surfaces can still occur depending on the substrate and on the metal or other conductive substance being electroplated thereon. Accordingly, certain bath additives termed brighteners have been developed because they provide a microscopic smoothing action which allows a finish that is bright and shiny to the naked eye. Although brightening mechanisms are not fully understood, one form of brightening is thought to occur through a process known as "leveling" whereby metal is removed from the high spots of microscopic ripples or irregularities without at the same time being removed from low spots or valleys (see Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 15, pp. 296-303, John Wiley & Sons, Third Edition).
Brighteners for nickel plating baths are generally lumped into two broad categories. The first is known as class I and includes compounds such as naphthalenedisulfonic acids, diphenylsulphonates, aryl sulfonamides, sulfonimides, etc. These brighteners generally improve brightness on pre-polished substrates but do not "build" luster on rough surfaces. The second is known as class II and includes certain metal ions and unsaturated organic compounds that are strongly adsorbed on the cathode. This class is fairly broad, a non-limiting list of commonly known representative members including propargyl alcohol, urea, pyridine, acetone, formaldehyde and water-soluble acetylenes such as dimethylhexynediol and butynediol (see Burns and Bradley, Protective Coatings for Metals, ACS Monograph No. 163, Reinhold, p. 220). Class II brighteners are thought to work through the leveling mechanism described above.
Most brightening formulations contain a combination of a class I brightener, which can be used in large amounts and which does not affect ductility, with a class II brightener which creates brittle and highly stressed finishes.
Such brightening formulations as described above, although useful for many applications, exhibit serious drawbacks with respect to nickel-phosphorus coatings, however. Nickel-phosphorus coatings themselves are highly desirable inasmuch as they are extraordinarily resistant to corrosion when the phosphorus content is above about 12 atomic percent. They exhibit very little leveling tendency, however, and appear to be affected only detrimentally by class II brighteners. That is, what little leveling can be obtained on nickel-phosphorus with a class II brightener generally occurs at the expense of reduced corrosion resistance.
Further, at or above a phosphorus level of about 12 atomic percent, nickel-phosphorus alloys are amorphous, i.e. they exhibit no defined crystal structure. Thus they are also largely unaffected by class I brighteners which operate by modifying the crystal structure of an electrodepositable substance as it forms on the substrate.
What is needed for this valuable class of alloy coatings is a brightening agent or combination of agents which will allow smoothing of a nickel-phosphorus electroplate. Such would improve the appearance of nickel-phosphorus coatings in their own right and also provide a smooth surface on which bright decorative overplating (e.g. with chromium) would be possible, chromium overplating on nickel-phosphorus as deposited by present methods generally appearing somewhat matte. Smooth nickel-phosphorus coatings would also be improved in longevity in applications involving metal-on-metal wear and abrasion. Such a brightening combination is the subject of this invention.